1. Field of the Invention
This invention relates to a shuttle hook driving device for a sewing machine, and more particularly to a shuttle hook driving device for a sewing machine of the type which employs a half-turn shuttle hook of the so-called DPS-type.
2. Description of Related Art
Shuttle hooks for a sewing machine are classified into a full-turn type and a half-turn type, and such half-turn shuttle hooks are classified into the type (hereinafter referred to as "DBS-type shuttle hook) in which a needle is disposed between an upper thread loop and a bobbin (that is, a needle drop position is disposed between a bobbin thread lead-out point of the bobbin and a beak of the shuttle hook) and the type (hereinafter referred to as "DPS-type shuttle hook") in which an upper thread loop is disposed between a needle and a bobbin (that is, a beak of the shuttle hook is disposed between a needle drop position and a bobbin thread lead-out point of the bobbin.
As used in the description and in the appended claims, the word "a standard sewing machine" means a sewing machine in which the needle drop position is disposed between the bobbin thread lead-out point of the bobbin and the beak of the shuttle hook.
In a sewing machine having the DBS-type shuttle hook, perfect stitches can be formed during a forward feed (that is, a feed from right to left as viewed from the operator side), while hitch stitches can be formed during a reverse feed (that is, a feed from left to right). Thus, this type of sewing machine has been widely used for forming general lock stitching.
More specifically, for general lock stitching, ordinary straight stitching must employ perfect stitches in view of the quality of the stitch. Because the stitching may become undone at the start and end of the stitching, hitch stitches are preferable to prevent the stitching from becoming undone. For this reason, the DBS-type shuttle hook is employed.
Incidentally, the sewing machine having the DBS-type shuttle hook is not suitable for omnidirectional stitching.
A sewing machine having the DPS-type shuttle hook is also known (For example, see Japanese Patent No. 99353), and is suitable for omnidirectional stitching because perfect stitches are formed during forward and reverse feeds. However, such a sewing machine has not yet been widely used.
Incidentally, this DPS-type shuttle hook can accommodate a change in thickness of work, and is suitable for situations where thick work or a thick thread is used.
FIG. 5 shows the construction of a drive device for a thread take-up lever and a needle bar in a sewing machine of the general type. This construction includes a main shaft (upper shaft) 1, a crank section 2, a counter balance 21, the thread take-up lever 22, a needle-bar crank 23, a needle-bar crank rod 24, the needle bar 25, and a needle 113.
FIG. 6 is a continuation of FIG. 5, and shows the construction of a shuttle hook driving device. This construction further includes a crank rod 3, an oscillating rock member 4, an oscillating rock shaft 4a, gears 5 and 6, a lower shaft 7, a shuttle race body 8, a driver 109, a shuttle hook (DBS-type shuttle hook) 111, a shuttle race ring 11, a bobbin case 100, and a bobbin 103.
FIG. 7 shows the conventional bobbin case 100 used for an ordinary half-turn shuttle hook. As shown in FIG. 7, the bobbin case 100 has an engaging member 101 for rotation prevention purposes, and a threading hole 102 is formed through this engaging member 101. A bobbin thread 104, fed from the bobbin 103 received in the bobbin case 100, is passed through the threading hole 102 in the engaging member 101.
The bobbin case is mounted on the front side of the sewing machine so that the operator can easily mount and remove it.
Since the sewing thread used in a sewing machine is basically of a Z-twist type, the shuttle hook needs to be rotated in a direction to twist the upper thread. More specifically, the direction of rotation of the DBS-type shuttle hook should preferably be clockwise as viewed from the operator side, and the direction of rotation of the DPS-type shuttle hook should preferably be counterclockwise.
In the general-type sewing machine using the above DBS-type shuttle hook, a curve A of lift of the thread by the thread take-up lever 22 and a curve B of take-up of the thread by the shuttle hook (DBS-type shuttle hook 111) are determined as shown in FIGS. 3 and 4, and these are thought to be the most ideal characteristics.
More specifically, it is well known that by providing the curve B of take-up of the thread by the shuttle hook (DBS-type shuttle hook 111) with respect to the curve A of lift of the thread by the thread take-up lever 22, good stitching free from improper thread distribution can be effected without causing undue noises and vibrations.
Next, the case where the DBS-type shuttle hook is used will be compared with the case where the DPS-type shuttle hook is used.
FIGS. 8A and 8B show a comparison between the case where the DBS-type shuttle hook is used and the case where the DPS-type shuttle hook is used. More specifically, FIG. 8A shows the DBS shuttle hook 111, a beak 112 of this shuttle hook, a needle 113, an upper thread 114, and an upper thread loop R. FIG. 8B shows the DPS shuttle hook 115, and a beak 116 of this shuttle hook. There is a problem that a distance (2) (FIG. 8B) between a needle drop point and a bobbin thread lead-out point, in the case of using the DPS shuttle hook, is longer than a distance (1) (FIG. 8A) between a needle drop point and a bobbin thread lead-out point in the case of using the DBS-type shuttle hook.
Namely, in the basic construction of the DPS-type shuttle hook 115, the distance (2) between the bobbin thread lead-out point and the needle drop point must be laterally shifted or offset by an amount equal to the width of the race of the shuttle hook plus .alpha. as compared with the distance (1) between the needle drop point and the bobbin thread lead-out point in the DBS-type shuttle hook.
As described above, the direction of rotation of the DBS-type shuttle hook is clockwise whereas the direction of rotation of the DPS-type shuttle hook is counterclockwise, and therefore the construction of the shuttle hook driving device must be taken into consideration in view of noises, vibrations and stitching.
More specifically, the directions of rotation of the DBS-type shuttle hook 111 and the DPS-type shuttle hook 115 are opposite to each other, and therefore if all of rotation transmitting parts of one of the two shuttle hooks are made symmetrically reverse in construction to the corresponding parts of the other, all those parts of one shuttle hook must be newly prepared. As a result, construction costs are greatly increased.
Therefore, it is necessary that the rotation transmitting parts used in the sewing machine with the DBS-type shuttle hook 11 be suitable for use in the sewing machine with the DPS-type shuttle hook as much as possible, thereby suppressing the increase of the cost of construction.
For this reason, it would be advantageous to have a shuttle hook driving device of the simple right-left inverted phase type in which in the case of the DPS-type shuttle hook 115, the oscillating rock member 4 is simply disposed in right-left inverted relation to the oscillating rock member for the DBS-type shuttle hook 111, where the top dead center of the needle bar 25 as a reference.
For such a shuttle hook driving device of the simple right-left inverted phase type, a curve of take-up of the thread by the shuttle hook (DPS-type shuttle hook 115) is represented by D in FIGS. 3 and 4. This curve D is much lower than the ideal curve B when the rotational angle of the main shaft (upper shaft 1) is in the range of between about 210.degree. and about 330.degree..
Namely, at that region where the thread take-up characteristics are lowered, the acceleration of the driver 109 becomes much higher than the standard during the take-up of the loop R of the upper thread 114 by the beak 116 of the DPS-type shuttle hook 115, so that noises and vibrations develop. In addition an improper thread distribution will develop, so that a line of stitches is interrupted.
Therefore, in the case of the DPS-type shuttle hook 115, the above-mentioned shuttle hook driving device of the simple right-left inverted phase type can be not used.
It would be advantageous to provide a shuttle hook driving device of the 180.degree. inverted phase type in which, in the case of the DPS-type shuttle hook 115, the crank section 2 of the main shaft (upper shaft 1) is disposed in up-down inverted relation to the crank section for the DBS-type shuttle hook 111 where the top dead center of the needle bar 25 as a reference.
For a shuttle hook driving device of the 180.degree. inverted phase type, however, a curve of the take-up of the thread by the shuttle hook (DPS-type shuttle hook 115) is represented by C in FIGS. 3 and 4, and this curve C is closer to the ideal curve B than the thread take-up curve D is, but is lower than the ideal curve B when the rotational angle of the main shaft (upper shaft 1) is in the range of between about 210.degree. and about 330.degree..
Namely, where the oscillating rock member 4 is disposed in right-left inverted relation to the oscillating rock member for the DBS-type shuttle hook, at that region where the thread take-up characteristics are lowered, the acceleration of the driver 109 becomes higher than the standard during the take-up of the loop R of the upper thread 114 by the beak 116 of the DPS-type shuttle hook 115. This causes noises and vibrations to develop, and also results in an improper thread distribution, so that a line of stitches is interrupted.
Therefore, in the case of the DPS-type shuttle hook, the above-mentioned shuttle hook driving device of the 180.degree. inverted phase type can not be used.